1. Field of the Invention
The present invention relates to a radio communication system, mobile radio communication apparatus and a method therefor, fixed radio communication apparatus and a method therefor, recording media, and programs, and more particularly to a radio communication system, mobile radio communication apparatus and a method therefor, fixed radio communication apparatus and a method therefor, recording media, and programs wherein degradation in system performance due to co-channel interference is reduced in environments where negotiation is not conducted between base stations.
2. Description of Related Art
Where a plurality of pieces of fixed radio communication apparatus (hereafter, referred to also as “base stations”) are installed and multi-cell operation is performed in a radio communication system in which multiple access is implemented by the TDMA (Time Division Multiple Access) method or the like, if a plurality of pieces of mobile radio communication apparatus (hereafter, referred to also as “mobile stations”) connected with different base stations are used on the same frequency channel in the same area, co-channel interference occurs and telephone calls are disabled in the area.
To cope with this, frequency repetition is used in common, wherein areas using the same frequency channel are located far away from one another and channels used are thereby limited.
FIG. 1 illustrates an example of frequency repetition with the frequency divided into seven. As shown in FIG. 1, seven frequency channels A to G allocated to the radio communication system are respectively allocated to seven blocks (hereafter, referred to as “cells”) marked with corresponding characters A to G.
As mentioned above, use of frequency repetition locates areas (cells) using the same frequency channel away from one another. However, in this method, the frequency resource allocated to the radio communication system is divided and the number of frequency channels one base station can use is limited to 1/number of divided frequencies, which impairs the efficiency of frequency utilization at the system level. In terms of this, therefore, it is preferable to reduce the number of divided frequencies as much as possible.
However, if frequency repetition with a low value set for the number of divided frequencies is used, the locations of cells using the same frequency channel are physically made closer to one another.
If 3 is taken as the number of divided frequencies, as illustrated in FIG. 2, for example, adjoining base stations (cells) do not use the same frequency channel but the distance between cells using the same frequency channel is shorter than in cases where 7 is taken as the number of divided frequencies (cases illustrated in FIG. 1). For example, the distance between cell 2-1 and cell 2-2 to which the same frequency channel A is allocated is shorter than the distance between cell 1-1 and cell 1-2 to which the same frequency channel A is allocated in FIG. 1.
Further, in fact, an area where the path losses with respect to a plurality of base stations using the same frequency channel are substantially equal is produced.
If a mobile station positioned in proximity to such an area transmits a signal to a base station with which the mobile station is connected, the transmitted signal is an interfering signal for other base stations using the same frequency channel.
For example, it is assumed that mobile stations 12-1 and 12-2 are positioned in an area where the path losses with respect to base stations 11-1 and 11-2 are equal (boundary between the cells (fringe)) and the mobile station 12-1 communicates by air with the base station 11-1 through a communication path (transmission route) 21-1 and the mobile station 12-2 communicates by air with the base station 11-2 through a communication path 21-4, as illustrated in FIG. 3. Further, it is assumed that the same frequency channel is used for the communication paths 21-1 and 21-4.
In this case, the mobile station 12-1 judges that the communication path 21-1 to the base station 11-1 is more suitable as a communication path the mobile station uses than the communication path 21-2 to the base station 11-2 in terms of attenuation and communicates by air with the base station 11-1 through the communication path 21-1. However, in fact, the path losses of both the communication paths 21-1 and 21-2 are substantially equal, and thus, when the mobile station 12-1 transmits a signal to the base station 11-1, the transmitted signal also reaches the base station 11-2 through the communication path 21-2. Since the base station 11-2 is in radio communication with the mobile station 12-2, the signal transmitted from the mobile station 12-1 through the communication path 21-2 is an interfering signal for the base station 11-2.
Similarly, a signal transmitted from the mobile station 12-2 to the base station 11-2 also reaches the base station 11-1 through the communication path 21-3, and thus the signal is an interfering signal for the base station 11-1.
As mentioned above, there is a problem that a signal transmitted from a mobile station to a first base station is an interfering signal for other base stations located in proximity to the mobile station.
Therefore, at the base station 11-1, signals from the mobile station 12-1 are always interfered with by signals from the mobile station 12-2, and at the base station 11-2, signals transmitted from the mobile station 12-2 are always interfered with by signals from the mobile station 12-1. As a result, decoding of the signals becomes unfeasible.
To solve this problem, a technique called soft handoff is used in common. Soft handoff is a technique wherein a mobile station positioned at the fringe of a cell simultaneously communicates by air with two base stations, for example, and the transmission power thereof is controlled according to information indicated by both the base stations, and has been already incorporated in the CDMA (Code Division Multiple Access) cellular system, for example.
However, to implement soft handoff, it is required for each base station to conduct negotiation with other base stations. For example, if base stations are operated by different owners or if base stations are not directly connected with one another through the same network, each of the base stations cannot exchange information required for soft handoff with other base stations and the implementation of soft handoff is difficult.